1. Field of the Invention
The present invention relates to an engine, and particularly to an engine having an in-cylinder injector injecting fuel into a cylinder and a port injector injecting fuel into an intake manifold or intake port and having a valve-opening-characteristic changing mechanism.
2. Description of the Background Art
Generally, a so-called dual-injection engine is known that has an in-cylinder injector for injecting fuel into a cylinder and a port injector for injecting fuel into an intake manifold or intake port, and that alternately uses these injectors according to the operating state of the engine to achieve, for example, stratified charge combustion when the engine is in a low-load operating region and homogeneous charge combustion when the engine is in a high-load operating region, or simultaneously uses these injectors at a predetermined fuel-injection ratio to improve such characteristics as fuel-economy and output.
Further, a dual-injection engine that is of the type as described above and that has such a supercharger as turbocharger is also known from, for example, Japanese Patent Laying-Open No. 11-351041.
This engine with the supercharger disclosed in Japanese Patent Laying-Open No. 11-351041 changes the ratio between the quantity of fuel injected from the in-cylinder injector and that from the port injector according to a supercharge pressure. Specifically, when the engine is in a steady-state mode in a high supercharge pressure region, the ratio of the quantity of fuel injected from the port injector is increased while the ratio of the quantity of fuel injected from the in-cylinder injector is decreased. When the temperature of the leading end of the in-cylinder injector increases to be equal to or higher than a predetermined value, the ratio of the quantity of fuel injected from the in-cylinder injector is made higher than that applied when the engine is in the steady-state mode. In this way, a homogeneous air-fuel mixture is generated in a combustion chamber to improve combustion efficiency and prevent deposits from accumulating in the in-cylinder injector.
Regarding a lean-burn engine with a supercharger that has the valve-opening-characteristic changing mechanism, it is considered to implement output torque control for a lean region corresponding to middle to low speed and middle to low load region, in view of improvements in fuel economy as well as reduction of exhaust emissions, especially reduction of NOx for example, by controlling the quantity of intake air according to the degree of opening of the throttle valve, and further by controlling the quantity of intake air through control of the valve lift or valve-opening duration and the close timing of the intake valve in the state where the throttle valve is fully open without pumping loss, and controlling the quantity of intake air through control of the pressure generated by supercharging of the supercharger, for example. It is expected that the above-described control of the quantity of intake air through control of the valve lift or valve-opening duration of the intake valve in the state where the throttle valve is fully open provides a large tumble flow, namely high tumble ratio of the intake air into the combustion chamber to extend the lean limit.
However, as seen from the relation between the tumble ratio and the valve lift or valve-opening duration of the intake valve shown in FIG. 3, substantially in proportion to the decrease in valve lift or valve-opening duration of the intake valve, the tumble ratio decreases. Thus, if the valve lift or valve-opening duration of the intake valve is decreased, the tumble ratio necessary for lean burn cannot sufficiently be achieved. Then, due to the decrease in tumble ratio, it becomes inevitable to set the control air-fuel ratio to be rich, as shown by the relation in FIG. 4 between the valve lift or valve-opening duration of the intake valve and the required air-fuel ratio. Consequently, a problem occurs that the fuel economy deteriorates and exhaust emissions, especially NOx deteriorate.